This invention relates to ceramic articles based on a three-dimensional interconnected mullite whisker crystal network and is specifically directed to the use of topaz crystals as a reactant in an in situ synthesis of mullite whiskers to densify and strengthen mullite whisker articles produced without conventional sintering. In particular, the invention relates to the reaction in situ of interconnected topaz crystals and silicon dioxide or topaz crystals, silicon dioxide and hydrated aluminum fluoride in shaped green bodies, especially extruded honeycombs.
Shaped ceramic articles which exhibit a high degree of thermal shock and creep resistance have a number of commercially important applications, for example, as diesel particulate traps, hot gas filters, molten metal filters, substrates for exhaust catalysts, catalytic substrates for ozone conversion, catalytic substrates for precious metal combustors (Catcom) used to power gas turbines and in metal matrix composites. Several major problems, however, hinder such uses of ceramics. First, ceramics are susceptible to damage, such as cracking, caused by thermal shock and/or creep. Another problem results from the fact that ceramic structures frequently are difficult to machine or to join, making it difficult to manufacture ceramic articles having complex shapes. This is especially true of thin-walled structures such as honeycombs.
Mullite is widely used in numerous ceramic applications and is a crystalline aluminum silicate of the approximate empirical composition 3Al.sub.2 O.sub.3.2SiO.sub.2. Mullite is characterized by a distinctive diffraction pattern, but mullite occurs in distinctly different crystalline shapes, the most common being prismatic cigar-shaped crystals, sometimes referred to as "acicular" form. This form of mullite occurs as "clumps" and may be obtained, for example, by firing clays. Mullite can also be synthesized as smooth elongated single crystals (whiskers). Whiskers have a significantly higher aspect ratio than the prismatic crystals in conventional mullite materials. Mullite can also be synthesized as highly elongate fibers. Generally, all forms of mullite have many of the known valuable properties of alumina, such as a high melting point and, in addition, exhibit other valuable physical and chemical properties. However, when mullite is formed as whiskers, the unusual strength associated with single crystals is PG,5 obtained. Topaz (AL.sub.2 SiO.sub.4 F.sub.2) is usually obtained as stocky, bar-like crystals which have a significantly lower aspect ratio than mullite whiskers. Topaz is characterized by a unique x-ray diffraction pattern and can be converted to various forms of mullite by reactions with silicon dioxide.
The art is replete with suggestions to use various whiskers, including mullite whiskers, to reinforce ceramics. Generally, in such use the whiskers are employed as discrete, nonagglomerated crystals which are formed into composites by conventional sintering technology. This inherently limits the content of mullite in the ceramic articles so produced and introduces potentially fluxing materials. Thus mullite articles based on composites including addition of discrete single whiskers cannot be used at temperatures as high as those that pure mullite can survive, and production of such composites necessitates handling fibrous material.
Various methods have been suggested in the prior art for the production of mullite in whisker form. Generally, these involve solid-solid reactions at high temperatures with evolution of gaseous by-product. Formation of mullite whiskers from the reaction of anhydrous aluminum trifluoride (or aluminum trifluoride and alumina) and silicon dioxide with a topaz intermediate is described in U.S. Pat. Nos. 4,910,172, 4,911,902, and 4,948,766, all to Talmy et al. The reactants pass through a topaz crystalline phase before mullite crystals are formed. According to the teachings of the patents, the solid reactants must be anhydrous and an anhydrous silicon tetrafluoride atmosphere must be present to form the mullite whiskers. In U.S. Pat. No. 4,948,766 a porous shaped preform containing the reactants and an organic binder is converted to a highly porous felt, exemplified in examples as small discs, the green bodies going through a topaz intermediate stage without isolation of the topaz intermediate. The initial green bodies are highly porous as are the felts.
Our copending application, U.S. Ser. No. 07/386,186, and now abandoned, is directed to an in situ chemical route for making porous mullite ceramic parts in near net shape from preformed precursors. The invention permits the formation of highly complex shapes such as thin-walled honeycombs, and it entails heating coherent green bodies containing a mixture of finely powdered hydrated aluminum fluoride and silicon dioxide in a molar ratio of approximately 12:13, along with a fugitive binder, while sweeping the volatile reaction products including silicon tetrafluoride and water from the bodies. The reactants form mullite according to the following equation: EQU 12AlF.sub.3.xH.sub.2 O+13SiO.sub.2 =2 (3Al.sub.2 O.sub.3.2SiO.sub.2)+9SiF.sub.4 +xH.sub.2 O (1)
X is suitably about 3 and can be as high as 9. At temperatures of about 600.degree. C.-800.degree. C. a topaz intermediate (Al.sub.2 SiO.sub.4 F.sub.2) is formed and above 890.degree. C. this reaction results in the formation of mullite whiskers. The topaz intermediate is not recovered in this process.
Mullite whiskers products obtained by reaction (1) are highly porous, typically about 80% porous, and thus they are relatively weak. However, many of the important potential commercial applications for high mullite whisker content shaped articles require higher strength while still possessing desirable micro and macro-structures.